In the semiconductor industry, nitrogen trifluoride has a variety of applications, including as a fluorine source for plasma etch, a cleaning agent for CVD reactors, and a dry etchant for semiconductors. Besides, nitrogen trifluoride is useful as a rocket fuel. With the development of the semiconductor industry, the nitrogen trifluoride gas has been in great demand. The increasing demand for a gas without any environmental pollution increased the demand for the nitrogen triflurode gas by geometric progression. As a matter of course, nitrogen trifluoride gas for use in these applications is required to be of high purity.
Nitrogen trifluoride can be prepared by various methods, including the direct fluoridation of ammonia, the use of plasma to react ammonia with fluorine, and molten salt electrolysis for electrolyzing NH4·xHF(x:1.8˜2.1) with ammonium fluoride or hydrogen fluoride used as a material.
In most cases, when using these methods, impurities, such as nitrous oxide (N2O), carbon dioxide (CO2), carbon tetrafluoride (CF4), dinitrogen difluoride (N2F2), etc., are concurrently produced in large amounts, so that purification is necessary to obtain highly pure NF3.
Particularly, CF4, produced as an impurity upon the production of NF3, is difficult to be removed from NF3 by general methods such as distillation, bulk adsorption, etc., because of the similarity therebetween in boiling point (NF3: −129° C., CF4: −128° C.), molecular size (NF3: 4.5 Å, CF4: 4.8 Å), and adsorption heat (Dipole moment: NF3: 0.235D, CF4: 0D). Accordingly, the refinement of NF3 generally requires the use of adsorbents, and active carbon, active alumina, and synthetic zeolite are well known as adsorbents. Active carbon and active alumina, however, is highly disadvantageous in terms of labor and cost because they must be frequently changed with fresh adsorbent or regenerated owing to its poor capacity for adsorbing impurities, which result in an inevitable loss of NF3 gas.
U.S. Pat. No. 5,069,887, yielded to Takashi et al., discloses a method of refining nitrogen trifluoride gas, in which synthetic zeolite having a certain porosity and water content is used as an adsorbent under predetermined temperature and flow speed conditions so as for NF3 gas to be adsorbed thereinto and then desorbed therefrom. This method, however, is disadvantageous in that an additional desorption process must be conducted, and the water content of the synthetic zeolite must be adjusted to be within a predetermined range. In addition, the NF3 gas refined by this method has a purity of 93% or less so that it cannot be directly applied to fields requiring high purity.
Another refining method using synthetic zeolite can be found in U.S. Pat. No. 5,069,690, yielded to Philip et al., which discloses the kinetic separation of gas mixture using gas-solid chromatography. In this method, hydrothermally treated zeolite having a certain porosity is used as an adsorbent and discrete pulses of a mixture of gases are passed through a bed of the porous adsorbent that kinetically adsorbs one gas more readily than the other gases, resulting in the selective separation of NF3 gas. However, the hydrothermal pretreatment for controlling the porosity of zeolite is difficult to conduct. In addition, the available time period of the zeolite, that is, the saturation time period, is too short to apply the zeolite method in the commercialized mass purification of nitrogen trifluoride.